Cutaneous squamous cell carcinoma (SCC) is the second most common diagnosed malignancy in Western countries, and its incidence is rising due to increased exposure to solar ultraviolet radiation (UVR) and population aging. While most SCC are successfully treated by surgery, they still represents a cause of significant morbidity due to scarring and disfigurement. More importantly, a percentage of tumors can metastasize, for which there are no tailored treatments. Thus, there is a need to gather a more complete understanding of the mechanisms perturbed in the initiation, promotion and progression of cutaneous SCC in order to develop new approaches for prevention and treatment of this malignancy. A large fraction of SCC shows hyperactivation of the protein Ras; unfortunately, Ras is an elusive molecule to inhibit pharmacologically, and due to its multiple functions, it may be too toxic for healthy cells to withstand such inhibition. Thus, finding upstream and downstream elements that participate in the oncogenic Ras pathways in SCC may reveal a more amenable target for chemoprevention and therapy. Studies from our lab have demonstrated a role for RasGRP1 in skin tumorigenesis through its ability to activate Ras, and support the concept that stimulation of RasGRP1 by intensified extracellular signals could contribute to the dosage of activated Ras necessary for malignant transformation and tumor progression in the skin. Given the fact that UVR is the most relevant carcinogen associated to cutaneous SCC, the goal of this proposal is to elucidate the potential participation of RasGRP1 in UVR-induced tumorigenesis. Using a combination of mouse models and in vitro approaches, we proposed the following aims: (1) Define the role of RasGRP1 in the acute response of keratinocytes to UVR, and (2) Determine the requirement for RasGRP1 in UVR-induced skin carcinogenesis, using a RasGRP1 knockout and a K5.RasGRP1 transgenic mouse models. The combined results should provide a strong test for our hypothesis that RasGRP1 is required for the tumorigenic effects of UVR in the epidermis, and set the basis for further studies on new drugs targeted to RasGRP1 that could be useful in chemoprevention and treatment of cutaneous SCC.